Conventionally, various techniques for correcting a camera shake have been proposed. The camera shake correction is a technique for detecting camera shake information (shake amount, shake direction, and the like) of an image sensing apparatus such as a digital video camera or the like by the photographer using an external sensor or image processing, and correcting any shake by moving a part of an optical system or extracting a part of an image to cancel the shake on the basis of the detection result.
As one scheme for detecting a camera shake amount, an external sensor detection scheme, e.g., a scheme for directly detecting a camera shake of the image sensing apparatus using an angular velocity sensor represented by a vibration gyro is known (see, for example, Japanese Patent Laid-Open No. 06-98246). Also, as another scheme for detecting a camera shake amount, a scheme based on image processing, e.g., a scheme for detecting a camera shake by detecting a motion vector of a screen from a plurality of sensed images is also known (see, for example, Japanese Patent Laid-Open No. 05-7327).
Furthermore, as a scheme for correcting a camera shake, optical camera shake correction schemes such as a method of correcting a camera shake by moving an image formed on an image sensor by moving a part of an image sensing lens system in a direction perpendicular to the optical axis, a method of correcting a camera shake by moving an image formed on an image sensor by placing a variable apical angle prism in front of an image sensing lens system and varying the apex angle of the variable apex angle prism, and the like are known (see, for example, Japanese Patent Laid-Open No. 06-98246). A characteristic feature of this optical camera shake correction scheme lies in that a broad correction dynamic range can be assured. However, this scheme requires mechanical components such as an actuator, optical element, and the like, and is disadvantageous in terms of cost.
On the other hand, camera shake correction based on a digital image extraction method which corrects a camera shake by preparing an image sensor relatively larger than an actually required image size, and extracting a part of an image so as to correct a camera shake from an image obtained from the image sensor in accordance with the camera shake amount is effective for moving image sensing or the like, and has an advantage in terms of cost since it does not require any mechanical components. Hence, this scheme has prevailed widely (see, for example, Japanese Patent Laid-Open No. 05-7327).
Actually, a video camera often senses an image without using any video light or flash even in dim night image sensing or the like. This is because in moving image sensing, the flash which can only instantly illuminate brightly is of no use. Also, the video light is not convenient to use since it requires much electric power and image sensing equipment becomes bulky and heavy. Therefore, in order to allow image sensing of a low-luminance object or low-illuminance object, a slow shutter mode that senses an image by prolonging the exposure time more than one frame time (the time for one frame or field) of a normal moving image is known (see, for example, Japanese Patent Laid-Open No. 06-90402).
However, since the aforementioned slow shutter mode assures a long exposure time by reducing the number of frames of a moving image, not only the motion of a moving image becomes unnatural, but also the camera shake correction based on the digital image extraction method cannot obtain a sufficient camera shake correction effect since a camera shake is more likely to occur during image sensing of one frame of a moving image. This is because the aforementioned digital image extraction scheme is a function of obtaining a moving image free from any camera shake since it corrects the motion between neighboring frames of a moving image by changing an image extraction range, and it cannot correct any camera shake that has occurred in one frame of a moving image.
In order to solve this drawback, a proposal which senses a plurality of images using a high-speed electronic shutter, obtains a camera shake correction effect by superposing images while correcting a camera shake by detecting a camera shake amount by vector detection, and obtains a sufficient image signal value has been made (see, for example, Japanese Patent Laid-Open No. 11-252445).
In the aforementioned conventional slow shutter mode, since charges are accumulated on an image sensor for a long period of time by long-time exposure, a high quality image with less noise can be obtained. By contrast, in the method of superposing a plurality of images for realizing the camera shake correction, noise in the image increases in proportion to the number of superimposed images, which causes degradation of the obtained image.